Modrons are native inhabitants of Mechanus and the purest examples of the lawful neutral ethos. They popped up here-and-there in a couple of Planescape adventures, but took centre stage in the The Great Modron March adventure anthology.
Daemons (or yugoloths, if you prefer) for Pathfinder Planescape that have no official Pathfinder RPG updates.
The mysterious, xenophobic bladelings of Acheron, updated for Pathfinder RPG.
Originally created for the Dragonlance campaign setting (and updated for v.3.5 in Sovereign Press’s Bestiary of Krynn, Revised), the eyewing became a staple creature in Planescape. Unfortunately when it was co-opted for Planescape, its description as a resident of the Abyss was not updated for the great wheel cosmology (the Abyss of Dragonlance is very different from The Abyss of the great wheel) so we ended up with the lawful evil eyewing as a native of The Abyss.
Devils (or baatezu, if you prefer) for Pathfinder Planescape that have no official Pathfinder RPG updates.
Demons (or tanar’ri, if you prefer) for Pathfinder Planescape that have no official Pathfinder RPG updates.
The arcanaloth from Dungeon Magazine #149, updated for Pathfinder RPG.
The dabus, the Lady’s caretakers of Sigil, updated for Pathfinder RPG.
Cambions have always occupied a unique place in Planescape. Although they’re considered “lesser” tanar’ri – only one step above dretches and manes – they play quite prominent roles in several Planescape adventures, and one cambion – Ely Cromlich – is the number two guy in the Doomguard.
Proxies as presented in Deities and Demigods are normal NPCs with a single divine rank bestowed upon them by a god. Any being with a divine rank is barred from entering Sigil by the Lady of Pain. The problem is that according to the original Planescape material, Sigil is positively lousy with proxies, since they’re the only way a god can take “direct” action in the Cage. Also, while proxies have been shown to be more powerful than most mortals, even high level ones, they have never been shown to have anything near the power of a being with a divine rank.